I'll agree with Russel Crowe. He really is an actor who is trying his hardest to be that leading man type, but he really lacks all the charisma of that man when trying to play that part. But he tries so hard to be suave and have that kind of look when his vocal abilities and look and appearance in general just keep him back. Watching A Beautiful Mind is to watch someone trying so hard to play that kind of part with little success.
The other that comes to mind is Renee Zelwegger. As hard as she tries too, she doesn't seem comfortable in her own skin. Her acting is her just trying to say a line without giving a twitch or glance that something is wrong and she is freaking out over nothing. And as Stanley Kauffmann once said, Neil Labute's greatest achievement may in fact be that he made her bearable to watch.