You shouldn’t have any problem with Crowe as a cowboy if you’ve seen 3:10 to Yuma.
http://www.youtube.com/watch?v=pVyTEGGp3Z0
or ‘The Quick and The Dead’.
But Roland isn’t really a cowboy in a traditional sense.
Kind of a mixture of cowboy and knight.
I think either Crowe or Mortensen would do OK with the part; I just think Isaacs would be better. By King’s own acknowledgement, the character and original storyline were born when he was watching Eastwood spaghetti westerns so it’s hard to argue against Clint as the archetype. However, in the subsequent books he gets increasingly descriptive in the physical details of Roland to include thick black hair (which later takes on some gray) and his, “bombardier blue eyes.” Roland was also high born and grew up in something of a courtly environ, although being turned over to Cort Andrus for rigorous training as a child, then grew to learn the ways of the road and a more knight-errant type existence, so he has a rough hewn exterior with a nobility at his core that is the result of nature and nurture...
Right below The Proposition.