Mysterious background characters are cool. Always drop them into adventures. You don't even need to do anything ever again with the vast majority of them. They add verisimilitude, making your campaign setting seem alive and populated with real people.
But when you need someone to suddenly be important, then that shoeshine boy they met three adventures ago can turn out to be someone highly relevant and interesting.
Commentary by memnarch (who has not seen the movie)
And the players will never trust background PCs again. Or at least, never trust that shapeshifters they kill are actually dead without personally disintegrating the body afterward. At least Babu can't split themselves into multiple beings. That really would be a nightmare of an adversary.
In retrospect, perhaps we should have been much more suspicious about the frog being. At the same time, it's not exactly surprising the new identity didn't get investigated more. There were at least four different scenes between landing on D'Qar and this one where the players are getting ready to attack the Peace Moon! I mean, why bother to get bogged down with a single background detail when there's a hyperspace cannon planet attack to prepare for? Especially when Jim's character is gathering up explosives.
Transcript
{begin flashback: on the Falcon, heading to Takodana}
Babu Frik: {voice-over} “O! great irony. From one torture to another: the Falcon had become my cage of misery! But with cunning hand I planted the seeds of calamity, that I might be cast free.”
[SFX]: Bang!
GM: Sparks fly from an overhead panel and smoke starts pouring out.
Yanni: What have you done to my ship?
{flashback moves to just after landing on Takodana}
Babu Frik: {voice-over} “We fell upon the soil of some nameless world, yet the accursed Wookiee’s instincts stirred. He sensed corruption in the heart of the ship, though its nature eluded him.”
Chewbacca: I say, given our recent concerns regarding Nute Gunray, I’d feel better if I stayed here and checked the Falcon’s antiviral systems are operating nominally.
Babu Frik: {voice-over} “I lingered in shadow, unseen.”
{flashback moves to D’Qar, as the Resistance prepare to assault the Peace Moon}
Babu Frik: {voice-over} “At the next port I grasped deliverance: an identity, a name, a form anew. But the wheel of the Galactic order had turned in my absence.”
Bollie Prindel: {in background} Excuse me... What planet are we on?
Zeppo: And Finn, get those explosives on board.
Finn: Explosives?
Resistance member: {in background} D’Qar. The Resistance base.
Bollie Prindel: {in background} “Resistance”... What year is it?
Resistance member: {in background} 34 ABY.
Bollie Prindel: {in background} ... What calendar is it?
{end flashback}
Poe: So your epic escape from the Sarlacc and the Falcon boiled down to becoming a weird NPC with no plot relevance?
GM: Hey, background NPCs are the safest place to hide. You guys ignore them all the time.
Poe: Fiendish!