Who co-starred with James Dean in rebel without a cause and giant then made his directorial debut easy rider even winning best new director at cannes?

The actor you're thinking of is Dennis Hopper.

He co-starred with James Dean in both *Rebel Without a Cause* (1955) and *Giant* (1956). He then went on to direct *Easy Rider* (1969), which was a critical and commercial success, earning him the Best New Director award at the Cannes Film Festival.