[SPOILER WARNING -- Episode III details in this post]
Now that the Star Wars saga is complete, we have seen that R2D2 is the only character who appears in all of the films without transformation. The fact that he knows the scoop on everything in the prequel trilogy makes him even more interesting in IV-VI. For instance:
Artoo's exciteability in IV takes on a whole new tone. When he says, "She's here, she's here, I've found her," he's talking about Leia as someone he's watched over for 20 years, and whose mother he was personally devoted to. When he gets spastic in the throne room at the end of the film, it reflects a whole lifetime of waiting for something good to happen.
His attitude toward Yoda in V is contextualized too. He's not just mad that this old crazy guy has come out of the swamp and started ransacking his master's stuff -- he knows exactly who Yoda is, and knows that the crazy-old-coot schtick is an act. He's trying to protect Luke from someone he sees as manipulative. He knows that this Jedi training stuff didn't go so well for Luke's father. So his worry when he's told to wait with the ship isn't just fear for himself -- it's concern about what Luke's getting into.
And here's a partial list of his pivotal accomplishments throughout the films:
- Saves the Queen's ship as it flees Naboo
- Alerts everyone of Obi-Wan's capture on Geonosis
- Saves Padme from getting burned up in the droid factory
- Provides critical support for Obi-Wan and Anakin to rescue Palpatine and kill Dooku (not necessarily all good results, but failure would have meant their deaths)
- Carries the Death Star plans
- Instigates the rescue of Leia from the Death Star
- Shuts down the garbage compactor
- Keeps Luke's ship flying long enough for Luke to destroy the Death Star
- Provides smokescreen cover for the escape from Bespin
- Repairs the Millennium Falcon's hyperdrive, allowing it to flee certain capture
- Conceals Luke's lightsaber until the critical moment on Jabba's sail barge
In pretty much every one of the films, there's some moment at which one or all of the principle charaters would be toast if it weren't for Artoo.
What a guy.