Robert Downey Jr. Is 'Challenging For The Right Reasons,' Says 'Due Date' Director

'All that Robert cares about is elevating the material at all times,' director Todd Phillips says.

Robert Downey Jr. boasts one of the greatest assortment of characters of anyone in Hollywood, but that doesn't necessarily make him the easiest man to work with. Among Downey's many fine qualities is his inability to be anybody but himself — except when in character, of course — which almost always makes for a very memorable interview.

It apparently makes for an interesting filmmaking experience as well, as "Due Date" director Todd Phillips recently explained.

"Robert is a very challenging actor," the director told MTV News. "He's just a challenging person, and I think he's challenging for the right reasons. It's always about elevating the material. Robert is never challenging about, "My trailer needs to be parked closer to the set" or "I need this type of food" or "Don't look in my eyes." All that Robert cares about is elevating the material at all times."

Phillips said he was never tempted to hold Downey back over the course of shooting "Due Date," particularly because the actor's charged personality pushed the director to get more creative in the filmmaking process.

"You never try to wrangle, really," he said. "The last thing we want to do on a comedy is rein people in, so I kind of appreciate that energy that [Downey and co-star Zach Galifianakis] have, that kind of connection — or lack of connection in a way — and I think it works for the movie. But it can be hard."

It certainly doesn't hurt that Downey has been singing Phillips' praises lately, having told MTV News that the "Due Date" director is his soul mate. The compliments aren't lost on Phillips, who confessed: "It's amazing. Having grown up and seen his movies — Julian in "Less Than Zero" and some of these iconic characters — just watching that as we grew up and now to be working with him, and having him go out and say that [I'm] one of the best directors he's ever worked with or 'Due Date' is the best experience making a film ... it's crazy, surreal and very humbling."

Check out everything we've got on "Due Date."

For breaking news, celebrity columns, humor and more — updated around the clock — visit