The first thing my husband said was "Wasn't that guy a russian in the last movie we watched?" Its funny how they could get away with actors playing all these different ethnicities back then without a second thought to it. Few of the people in this movie attempted an accent. Both of the women who 'married' Pancho had very blue eyes. I just thought that took away from the story line for me.
re: actors playing different ethnicities. Hollywood still does this! Take a look at actor Cliff Curtis' resume. He's a 100% Maori from New Zealand, yet has played a Kurd, a Mexican, a Colombian (three times), an Indian and a Puerto Rican. And they're only the ones I know off the top of my head. He's been in other major movies (e.g. Live Free or Die Hard and Runaway Jury) where his nationality/race isn't mentioned but I'd wager it wasn't Maori. About the only race he hasn't played in a Hollywood movie is Maori!
Basically whenever a role for a dark skin comes up, Hollywood calls up Curtis cause he fullfils the description. Luckily for Hollywood he's also brilliant actor.
I think a certain amount of playing different ethnicities is permissible, providing one avoids anything crass like "blacking up". I think it's ok for, say, a Dane to play a German. Or a Brit to play a Yank or vice versa.
Well, yes, I agree with the OP there were few if any speaking roles for Mexicans in this film, but as far as ethnic casting goes, it's a lot better today than it was in 1968.