In acting, all characters are given a "backstory" which isn't necessarily shared between the actors or anyone else except with the creator (Matt Weiner). That backstory will color or influence the character's "reaction" and interaction with other characters.

Yes, Bryan Batt was probably informed of his character's homosexuality. Just as Jon Hamm was informed from the very beginning of the series that his character was in an assumed identity though neither we, the audience, or probably January Jones (Betty Draper) knew that until Don Draper told Rachel and then it was out in the trade papers. Nor did anyone else know that Peggy Olson was pregnant (except Elizabeth Moss) in the series until the last episode. Weiner is very guarded about each character's history/path except with the individual actor playing the character. The rest of the actors/characters aren't privy to it any more than the audience is until "it" is revealed in the script.

Bryan Batt said in his interview on the website that he researched his role by talking to gay men who were that age at that time. Almost without exception, to be gay meant to be closeted in those days, at least in the business world.

Thank you, jamm54 and chopin47. Very interesting. I did not realize that Don D. had told Rachel about his assumed identity! Yes, I know homosexuals of that time would mostly have been in the closet. I worked with 2 gay men at that time whom I considered to be very good friends, but even to me they kept their private lives very separate from their work lives. They are gone now, bless them. They both had wicked senses of humor - oddly enough, they neither one liked the other - perhaps it was their 20+ age difference.

@zerelda: Don didn't tell Rachel about his assumed identity. What he did tell her was that his mother was a prostitute who died giving birth to him, and that his father died from a horse kick to the head when he was 10 years old. And, that Dick/Don was raised by his stepmother (his father's wife) and the new man she took up with after Mr. Whitman's death (Dick/Don's father).