I watched a F.R.I.E.N.D.S episode and hehe... Dan Castellanata guest appeared in it.  I am not surprised if you don't know him by his name, but I am sure you would know him by his voice.  Well, maybe not...

Click image to see it enlarged.  Dan Castellanata (voice actor for Homer Simpson) guest stars in F.R.I.E.N.D.S as a zookeeper who tells Ross the truth about Marcel the monkey.

Castellanata is the voice actor behind America's favourite dad.  His name is Homer Simpson .  In the F.R.I.E.N.D.S episode, Castellanata is a zookeeper.  In that episode, Phoebe is hired to sing children's songs.  Joey gets an obessed Brooke Shields as his mega-fan who can't distinguish real life from TV drama.  Also, Ross finds out from the zoo manager that his monkey, Marcel, has died. 

: in reality, David Schwimmer dislikes the monkey very much.

Anyway, as Ross is glooming over the loss of his pal and angry at the zoo management for not informing him about Marcel's death, a zookeeper who is sweeping the floor comes up to Ross saying that he knows the truth and the truth is that Marcel is still alive.

The zookeeper tells Ross that Marcel is currently in the Entertainment business.  Afterwards, he asks Ross how much is this information worth to Ross.  Ross replies that he has already told him everything.  I am just disappointed that he didn't say "D'oh!"

[Scene: San Diege Zoo.  Ross is still at the monkey cages. A zookeeper is sweeping.]

Zookeeper: Meet me in the nocturnal house in 15 minutes.
Ross: Uh... hey look, I don't really enjoy being with other men that way.  But, um... zoo dollars?
Zookeeper: It's about your monkey.  It's alive.

[Scene: San Diego Zoo Nocturnal House]

Zookeeper: Ahh... the bat.  Ambassador of darkness, flitting out of his cave like a winged messenger, sightless spectre of the macabe.
Ross: Buddy, my monkey?
Zookeeper: Oh, yeah, right. There was a break-in, few months back, inside job. Your monkey was taken.
Ross: Oh my God.  But the zoo told me that my monkey was dead.
Zookeeper: The zoo!  Do you believe everything the zoo tells you?
Ross: Actually, that's the only thing the zoo's ever told me.
Zookeeper: Of course they're gonna say he's dead.  They don't want the bad publicity.  It's all a great big cover-up.  Do you have any idea how high up this thing goes?
Ross: That guy Lipson?
Zookeeper: Lipson knows.  Do you have any idea who else knows?
Ross: No, I... I only know Lipson.
Zookeeper: Hmm, Lipson knows huh?  Ahh... hello Mr. Opossum, enigma of the trees, upside-down denizen of the night, taunting gravity with...
Ross: Buddy, my monkey, my monkey.
Zookeeper: Word on the street... well, when I say street, I mean those little pretend streets they have here at the zoo.
Ross: Of course.
Zookeeper: Your monkey found a new career, in the entertainment field.  That's all I know.
Ross: This is unbelievable.
Zookeeper: So, what is this information worth to you, my friend?
Ross: Are you trying to get me to bribe you?
Zookeeper: Maybe...
Ross: But you already told me everything.