Actors are indeed people. They do indeed have off days. They could be sick, have trouble at home, any number of a hundred reasons for them to be less than pleasant.
As someone who has met a fine number of actors, I tend to dismiss when I hear a singular "bad" encounter with someone.
I only consider such when they're repeated, or I've experienced the actor multiple times.
William Shatner is a great stage personality, but is indeed quite distant and reserved when one-on-one. You can't fault the man for it too much, with the time and volume of events he's done. But when you hold him up to the rest of his cast mates, it's clear he's just not that "giving" on a personal level.
Patrick Stewart is much the same. On stage, he can be lovely. One on one? Oy. When I met him, even his on-stage self was a bit of a prick, to be blunt. My good friend Bill has met him more recently, and found him to be neutral. But he knew well enough not to try anything but "business" of autographs.
Nichelle Nichols is one not mentioned much, but she can be a royal snot. I've met her once and found such true. Heard it from others. She can be more ammeniable -- friend of mine actually got her to sign a phot for me with "Hailing Frequencies Open" -- which she obviously hates to do, lol. But I also know of a fan I met when seeing her, who had a fully signed TOS cast photo -- including Gene! -- and only lacked her because she refused to sign it. Why? Her head was partially obscured by Nimoy. The guy even offered to make out a $500 check to the charity of her choice, and she still refused. Which is just silly. If your own ego at not being perfectly in frame makes you a snot, you know, shame on you. If you won't overcome that and sign your name for a generous donation to a charity? Wow. Sad.
Actors also have phases, I might add, in addition to being subject to normal moods.
I met Rene Auberjonis the month after DS9 went off the air. He was so friendly and personable, as was Nana Visitor, I was shocked. It was one of my first conventions, had only done 2 or 3 others. I met him exactly one year later, the summer he did 'The Patriot' with Mel Gibson and relative unknown Heath Ledger? Wow. He was distant, dressed all spiffy, and surrounded by an entourage that 'protected' him. At the convention in question, we happened to all get thrown outside that hot August day. It was Philly, where the Republican National Convention for President was at the same weekend, city. Well the Governor of Illinois sent some alarm clocks to his delegation, and some dip-wad thought "bomb" and we're all outside for 3 hours.
During this, Robbie McNiell and Armin Shimmerman worked their asses off entertaining us, posing for pictures, telling stories, listening to the fans. They went above and beyond, and they didn't have to. They could have cloistered off under an umbrella and been "celebrities" but they really swung into action. Rene? He *DID* stand off cloistered by his entourage, segregated, refusing to let anyone near him. It was such a radical departure from only 12 months earlier.
While off topic, in terms of favorites and nicest?
Avery Brooks gets the award for being the "biggest star" who is nicest. He is so deep and philosophical, yet so down to earth and friendly. He takes the time to shake your hand, ask your name. He's a man of the people, very casual. He also levels with you: he does these shows to say "hello" to the fans. He doesn't promote any projects unless you ask him directly, and even then, he tends to keep that brief! He genuinely enjoys experiencing the diversity of the fans and to see what Star Trek means. I'm left in awe whenever I meet him, because he doesn't have to do that.