Faking chemistry with your male co-star in a film is impossible, says Jennifer Aniston.

The former 'Friends' star who was previously married to Brad Pitt has starred alongside some of Hollywood's hottest men like Clive Owen (in 'Derailed'), Owen Wilson ('Marley and Me') and former beau Vince Vaughn ('The Break-Up').

Aniston, recently seen in romantic comedy 'He's Just Not That Into You', believes it is vital for the two lead actors in a film to connect. But, she says, making that connection is sometimes difficult.

"You can't buy chemistry. You can't train it," she explained in a recent interview.

"On the first day of rehearsal it's like: 'This is good.' You know it when it happens and boy do you know it when it's not there."

Although Aniston has had the most acting success of all the 'Friends' stars, she insists her aim is still to become a director.

"I'm still working on becoming a director," the 40-year-old said.

"I'm just waiting for the right thing. When you get to a place in your career where you're like: 'OK, I feel like I can breathe and I can make some choices that creatively stimulate me.'

"I don't know if acting is always going to be it, but I love it."