For an Oscar-winning director whose fearsome reputation preceded him down the decades, age appears to have done much to mellow William Friedkin – in the flesh in Dublin he's as laidback as he is funny. On screen, however, it's a different matter, and the 76-year-old French Connection and Exorcist legend's new movie Killer Joe is a ferocious piece of Texas noir. Adapted from the Tracy Letts play of the same name, it tells the story of a redneck family (Emile Hirsch, Thomas Haden Church, Juno Temple, Gina Gershon) whose get-rich-quick scheme finds them seeking the services of Joe Cooper (Matthew McConaughey), a contract killer who also happens to be a cop. Anyone who sees the movie will never look at the excellent McConaughey in the same way again, and hopefully his director will be giving us such big screen surprises for many years to come.

Harry Guerin: If I hadn't known that you had directed Killer Joe, and if someone had asked me after watching it what age I thought the director was, I would've said 30 - it has that hunger.
William Friedkin:
[Laughs] Well, I am around 30. If you consider guys who are over a hundred-years-old [are] still making films, I'm closer to 30 than they are! I'm in the ballpark, as they say.

In the book Easy Riders, Raging Bulls you're quoted as saying "Film in America is a young man's game". Did something change your mind?
No, I maybe am young at heart, or I have peculiar tastes in the films that I've made. And my tastes drift along the lines of [Killer Joe] writer Tracy Letts' script. We see the world through a similar prism.

Matthew McConaughey gives arguably the performance of his career to date in Killer Joe. Given his rom-com history, had you reservations about casting him?
None whatsoever. He's a very good actor. The problem - it's not a problem really for him - is he's a very good-looking guy by Hollywood standards. And when an actor is fortunate enough to become a star in Hollywood because of his looks they don't want you to act; you just need to show up and take off your shirt and make love convincingly to the leading lady. And he did that for many years and made a fortune doing it, and so who could blame him?! I mean, it's a good life with a big pay cheque at the end of the rainbow.

But he's much more of an actor and he has - beginning with Killer Joe and I think even before that The Lincoln Lawyer - taken control of his life, of his career and decided to do acting roles that challenge him, even if they're not the starring part. When I met him I recognised that he was in that phase of his life. He comes from that part of the country [Texas] where this film is set and not only is his accent perfect, but his attitude about the people there... He knows, he understands and he's not judgemental about them.

And did he have reservations about making the movie?
He did originally. What I didn't know before I got a call asking if I would meet with him is that when he first read the script he thought it was... His word was vile. He said he damn near threw up! And then he started to think about it, and it occurred to him that it was not only true to the human nature of those characters but it was funny. Absurdly funny. So he went back to it and read it again and thought, 'Well, I'll take a shot at this'. He was glad I was directing it; I think he felt he'd be in good hands and that I would help him.

Do you think your cast had preconceptions about you before they started working with you? Did anyone say 'I thought you'd be more fiery' or 'I thought you'd be more laidback'?
If they did, I wouldn't have paid attention to it. They may have said such things, but, y'know, that's not the sort of thing that penetrates. I really don't give a damn!

The job of a director consists of first, finding the material that you feel compatible with. Second, casting it well. And once you get past those two things you're about 75% done. Then the next thing that a director has to do is create an atmosphere wherein the actors and the technicians who actually make the movie can feel free and comfortable to create, to add their own ideas and notions to the film and not feel that the director is going to be judgemental or say, 'No, that's not it!' And that's not what I do. If I have ever done it - and I may have! - it's because I miscast the part. But those are the main obligations of a director. Forget all the other creativity, where you put the camera, how you edit the shots - those are creative responsibilities. The practical responsibilities are what I just said: choose the material, cast it right, provide an atmosphere where the actors and the crew can do their best work.

There's a line in Killer Joe, "Nothing's worse than regrets". In terms of movies, are there ones you wished you had made? That you were offered and turned down?
[Pauses] No. I can't think of one. I've only made 16 films in about 45 years. I worked on this for a year-and-a-half from the time Letts sent me the script to... It hasn't opened yet and I'm doing publicity for it. I worked on The French Connection - from the beginning when we had no script just a story - for two-and-half years before I delivered that and it was finished. Then I'd do all the foreign versions – supervise the dubbing and the subtitling. I worked on The Exorcist for well over three years. So those three films alone took over eight years of my life.

Is there a project you would still really like to do?
I would love to do my version of the Jack the Ripper story. That's a film that I still have in me and the rights that I had to this one particular account, which I find believable, are really entangled. They were from the beginning. That's the only story that I really have a feeling for.

So much good drama is on TV now. I always thought you would've done a great job directing episodes of The Wire.
I didn't respond to The Wire. People think it's genius; I found it a tough slog. I've directed two episodes of CSI for [star] Bill Petersen. I did his last episode and then they invited me back to do the two-hundredth episode, which I did. I did, several years ago, the new Twilight Zone. [It was] a film called Nightcrawlers - a short - which they still show all the time – that's probably 25 years ago. I did an episode for Tales from the Crypt. I've done several things that I liked.

There are only two or three series that I've seen that I really like. I love The Sopranos and 24. And then recently a show called Boss. That's about it. Well, there was a show called Damages... But I don't watch a lot of TV.

So what movies by young directors have you really rated in the last few years?
I like the Coen brothers' work – all of it pretty much, some better than others. I like the work of Paul Thomas Anderson; I think he's only made four films. Wes Anderson, I think, is a very interesting director. Not too many others come to mind of the younger directors - it's because I don't see a lot of films. Y'know, I still like some of the masters who are still working: Costa-Gavras, Claude Lelouch's films, Alain Resnais... Just a handful of guys who are still working, not at their peak, but I'll still go and see a film that they've done. This guy Michael Haneke, the Austrian guy who does a lot of his films in France, I know him and I think he's the real deal. I loved Caché [Hidden]. That's a movie.

Killer Joe has received the strictest certificate, 'NC-17' - No Children Under 17 Admitted - in the US. Back in 1980 your Al Pacino-starring thriller Cruising had big problems with the censors and you had to cut it to avoid the strictest cert of the time, an 'X', and get an 'R' - Under 17 requires an accompanying parent or adult guardian - rating. Do you think there's more leeway now when it comes to getting certs?
Only to a major studio. For example, if Cruising had not been made by a major studio, which it was - it was made by United Artists - it would have just been given an 'X' and that would be that. But the fact that it was a major studio meant that I had to curtsy and bow before Her Majesty The Ratings Board and make some cuts. I went back 50 times with that picture before they were just worn down and gave it an 'R'. But originally they thought that this [Cruising] could never possibly achieve an 'R'. And that's the situation with this [Killer Joe]: there's nothing I could do with this film to cut it so that it would get an 'R'. Nothing. I would have to do what the generals in Vietnam said about why we were over there: we have to destroy this country in order to save it. That's what I would've had to do to Killer Joe to get an 'R' - destroy it.

There's a wonderful phrase that Harry Truman used to use. People used to shout out at him: 'Give 'em hell, Harry!' And he used to say: 'I don't give 'em hell; I just tell the truth and they think it's hell!' And that's my attitude: I'm not trying to shock them; I just tell the truth and they think it's shocking!

Killer Joe is in cinemas from Friday June 29.