1) The characters of Ted, Marshall, and Lily are based on the show’s creators, Carter Bays and Craig Thomas, and Thomas’s college sweetheart and wife, Rebecca.
2) Barney’s character was originally described as a “Jack Black, John Belushi type,” but as soon as Neil Patrick Harris auditioned for the role, the creators got rid of the description.
3) During his audition, Neil Patrick Harris acted out Barney playing laser tag. He did crazy dives and rolls, even slamming into the creators’ desk and knocking everything over.
4) Jason Segel was Bays and Thomas’s first choice for the role of Marshall. They were both huge fans of Freaks and Geeks.
5) Casting director Megan Branman spotted Cobie Smulders doing a bit part on a drama series while flipping through channels. She immediately told the creators she had found their Robin.
6) Jim Parsons originally auditioned for the role of Barney.
7) Jennifer Love-Hewitt was originally supposed to play Robin, but she booked Ghost Whisperer instead.
8) Britney Spears reached out to the creators about wanting to be involved in the show.
9) The creators planned to make Victoria the mother if the show was cancelled during Seasons 1 or 2.
10) In “Something Blue,” the engagement that happened behind Robin and Ted was real. The extras were related to one of the writers and big fans of the show.
12) The idea for the “Slap Bet” between Marshall and Barney came from Bays, who would make slap bets with his high school friends.
13) The running joke “Have you met Ted?” was actually started by Bays and Thomas’s boss at Letterman. He would say, “Have you met Carter?”
14) Cobie Smulders, Alyson Hannigan, and Neil Patrick Harris’s real-life husbands all appeared on the show more than once.
15) The cast had the tradition of walking into table reads altogether. Jason Segel suggested it because that way they would all get there early and could eat the free breakfast provided.
16) Thomas and Bays decided to use two different actors for Ted — Josh Radnor and Bob Saget — so viewers would understand that Ted went on a transformative journey and wasn’t the same person he used to be.
17) Barney and Robin’s relationship wasn’t planned from the beginning. The creators came up with the idea after seeing the chemistry between Neil Patrick Harris and Cobie Smulders.
18) In the episode “Bad News,” Jason Segel didn’t know Marshall’s dad was going to die until they shot the episode. When Hannigan said her line, “He didn’t make it,” we see Segal genuinely reacting to the news.
19) Neil Patrick Harris drank so much Red Bull both on-screen and off while playing Barney Stinson that the company gave him a lifetime supply.
20) The actors who played Ted’s children, David Henrie and Lyndsy Fonseca, filmed their final scene in which we learn who Ted ends up with during Season 2. They were then sworn to secrecy.