John Slattery says losing Philip Seymour Hoffman was ''tragic''.
The 51-year-old 'Mad Men' star directed the late actor in 'God's Pocket' only a few months before he died of a drug overdose in February this year, and he admitted the loss meant his memories have been ''ruined''.
He told The Telegraph magazine: ''Losing him was tragic, and it ruined a lot of it. The experience, not the film.
''He was in love with the film. But afterwards, it took on this mantle of something it was never meant to carry, which was too bad.''
The film - which sees Seymour Hoffman play Mickey in one of his last roles - was previewed at the Sundance Film Festival and Slattery admitted he was nervous about how well it would be received.
He said: ''I was scared out of my mind, but [Philip and I] sat there laughing our heads off.
''He hadn't sat and watched a movie he was in for 10 years at a [public] screening. He was so pleased with it.''
The director - who had wanted the actor for the movie from the start - praised the late star for his thoroughness and inquisitiveness.
He added: ''He relentlessly wanted to know why: why Mickey stayed, what he was after, what he was afraid of in his relationship to Jeannie. Mickey is a non-verbal guy, so he was collecting the knowledge of the guy so he could just be, and communicate.
''He was always curious as to why this guy fought so hard to be a member of this community where people were constantly reminding him that he wasn't one of them.
''He seemed so unaware of all that but then when we edited, he'd go, 'No, there's a better take where my arm is up on the counter,' and you realise that he knew exactly where he was in the room, where the camera was all the time.
''I never worked with someone who was so thoroughly doing what he was meant to do.''