WARNING: This post contains SPOILERS for Avengers: Endgame.
Marvel Studios' Avengers: Endgame features the surprise appearance of real-life Jarvis, but for fans not well-versed in the larger Marvel Cinematic Universe, Edwin Jarvis is played by James D'Arcy. Jarvis (or J.A.R.V.I.S.) was originally introduced in the MCU during 2008's Iron Man, where the "character" was Tony Stark's (Robert Downey Jr.) personal user interface computer system. Among other things, Jarvis helps Tony in his workshop to build the Iron Man suit, then becomes the U.I. inside the suit itself. Over the course of the films, Jarvis becomes integrated with the Mind Stone and evolves into the Vision android in Avengers: Age of Ultron. Paul Bettany, who voiced Jarvis in the MCU movies, also plays Vision.
However, as the Marvel films saw Jarvis evolve into Vision, the television side of the MCU provided Jarvis' back story and revealed the interface was inspired by Howard Stark's (Dominic Cooper) personal butler Edwin Jarvis. In the Agent Carter prequel TV show, Jarvis appeared as a main character, assisting Peggy Carter (Hayley Atwell) in her various missions for the Strategic Scientific Reserve. For Avengers: Endgame, Marvel dug into its own TV roots for a special Jarvis appearance.
When Tony and Steve Rogers (Chris Evans) travel back to 1970 in order to get the Tesseract/Space Stone and more Pym Particles in order to return to the present, Tony runs across a younger version of his father, played by John Slattery. The two share some touching moments talking about fatherhood, and Tony gets some closure to his troubled relationship with Howard. At the end of this sequence, Howard gets into his car, and standing beside it is his butler Jarvis, played by Agent Carter's own James D'Arcy.
MCU fans who have only watched the movies - or, at least, those who didn't watch Agent Carter - may not realize the connection between Howard's butler and the larger universe. But in fact, D'Arcy appearing as Jarvis in Avengers: Endgame is the first instance of an actor cast specifically in a Marvel TV role making the jump to the MCU movies. Fans have been calling for the likes of Daredevil and the Agents of S.H.I.E.L.D. to team up with the Avengers, but while Endgame didn't deliver on those particular fan wishes, the movie does acknowledge at least one TV character.
Jarvis' appearance in Endgame also further confirms the character's life-long tie to the Stark family. Although Howard loaned Jarvis out to Peggy Carter in the 40s, he's still very much a trusted ally to Howard in the 70s. It further cements why Tony's user interface is named Jarvis, because Jarvis was such an important part of Howard, then Tony's life. Bringing Jarvis to life, and having Agent Carter's D'Arcy play him, is a good way to honor the character as well as the TV side of the MCU in Avengers: Endgame.