Charlie, every now and then, goes to a psychiatrist played by Jane Lynch. In "My Puppy's Dead", he and Alan go home after one visit and watch Glee, to which he responds "that tall blond in the red track suit is freaking me out".
In "People who Love Peepholes", Alan says that "when I was was in high school, I was dating a poster of Molly Ringwald". Before Two and a Half Men, Jon Cryer was most famous for Pretty in Pink, where his character, Ducky is hopelessly in love with Ringwald's.
In the season 12 premiere, Alan dresses as Ducky for Halloween.
In "A Giant Cat Holding A Churro" Alan confronts his girlfriend Lindsey, played by Courtney Thorne-Smith, with her playing in a softcore porn movie when she was younger. She then tells that the producer had said he could get her on Melrose Place. Courtney Thorne-Smith was a Melrose Place regular for the first five seasons.
A somewhat atypical example. When Alan starts believing himself to be Charlie out of grief, he ends up institutionalized, and imagines he's calling up hookers like Charlie did. Upon "hanging up", he utters Charlie Sheen's own infamous post-firing catchphrase "Winning!". (This is as much an allusion and a jab at Sheen, of which season 9 has plenty).
In "You Do Know What the Lollipop Is For", Walden comes down the stairs, and then is taken aback when he can't remember why he came downstairs. Alan suggests "Maybe too much (imitates inhaling a joint) oh hey, dude, where's my car?" Ashton Kutcher starred in the 2000 stoner comedy Dude, Where's My Car?.
Missi (Miley Cyrus) in the same episode tells Jake on the beach an Orphaned Punchline of "And that is why you don't smoke pot in church". This may be a reference to Cyrus' 2011 salvia incident (and her self-deprecatingReal Life nickname of "Bob Miley" due to the same incident).
In "How to Get Rid of Alan Harper", Walden's girlfriend proposes a game of "bang, marry, kill", with Angelina Jolie, Mila Kunis and herself. Later, when she cancels her date with Walden and leaves, he says: "Better hope I don't run into Mila Kunis!" She responds: "Yeah, like that's gonna happen". Mila Kunis is Ashton Kutcher's girlfriend in real life.
"Lan Mao Shi Zai Wuding Shang", where Mila Kunis guest stars has as Vivian, has several.
When she introduces herself:
Walden: That's a beautiful name.
Vivian: Thank you. My parents got it from that '70s show. What was it called? Maude!
When Walden invites her to stay the night because it really started to rain, she says: "It's just a little water. I'm not gonna melt. (spoilers for a then-recent film of hers)It's not like I'm the Wicked Witch of the West."
When they talk about dating:
Vivian: So what's your deal Walden? You're a good looking guy, rich, Malibu beach house... I bet you only date hot actresses.
Walden: God no, I'd never date an actress. They're all crazy.
Vivian: I'm sure they're not all crazy.
Walden: Trust me, they are.
Vivian: I don't get this whole fascination with celebrities anyway. I mean who cares who's dating whom or who's engaged to who, or who has a sex tape that no one will ever, ever see?
One episode features Ken Jeong as a nurse. Jeong is a licensed physician.
Casting Gag: In Season 11, Alan begins dating a woman played by Kimberly Williams-Paisley. Later her ex-husband appears who is played by...Brad Paisley
Creator Backlash: Angus Jones (who played Jake) left the show at the end of season 10 after he converted to Seventh-Day Adventism. He now treats it as an Old Shame, claiming that it is immoral and that he had been a "paid hypocrite" by starring on it.
Creator Breakdown: The circumstances surrounding Charlie Sheen's exit are an explosive example.
Money, Dear Boy: A variation; the main reason why Chuck Lorre pushed the network to commission a pilot was because co-creator Lee Aronsohn would cease to be eligible for WGA health coverage if one of his scripts wasn't put into production very soon. Fortunately for all concerned, it turned out that the network quite liked the pilot...
The actress who plays Chelsea, one of Charlie's few meaningful relationships for a long time (some would say too long) had already appeared in other roles in previous episodes, including the Pilot.
April Bowbly, who later played Kandi, made an earlier appearance playing a woman in Charlie's shower, called "Bubbles." It is possible Bubbles was Kandi, as Kandi shows up two episodes later and acts just like her (April Bowbly is credited as "Kimber" in that episode, even though she's never adressed as such in the entire episode).
Jenna Elfman appeared in a two-part episode in the first season as a woman named Frankie. She later appeared as Dharma in the Season 9 opener.
Judy Greer, who plays Walden's wife appeared in the fourth season as Herb's sister, Myra.