He might puzzle for a second over the tiny grey unitard within before recalling that this was his "costume" when he appeared in dad's film, Dawn of the Planet of the Apes, playing a toddler ape at school.
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"They made him a little mo-cap [motion capture] suit," says Reeves. "It was very cute."
Once the special effects were added, no-one but mum and dad would be able to tell which child was theirs, but it is a measure of the lengths to which the Apes films go to make everything seem real that the scene began with actual children.
"I didn't want an idealised sense of a school of chimps, I wanted them to be just as chaotic and disinterested in learning as real two year olds might be," adds Reeves.
Dawn follows 2011's Rise of the Planet of the Apes (global gross £280 million), not the first but certainly the best successor to the original Planet of the Apes. The new films are prequels to the 1968 classic starring Charlton Heston and Roddy McDowall and explain how the Planet came into being. Taking the lead again in Dawn is Andy Serkis, playing Caesar, the ape raised by humans who strikes out on his own.
Reeves was two when the 1968 film appeared. His introduction to the world of the apes came with the television series, before he discovered the original picture. "I had all the dolls, and records, comic books, it was an early obsession of mine."
Now he has had the chance to play with the ultimate train set that is motion capture technology. Capturing the performances of real actors and translating those into computer generated apes is the job of visual effects firm Weta Digital. It is a complex process, but one with a simple aim: believability.
Reeves says: "My hope is that audiences, even knowing about the visual effects, will say, 'Wait a minute. There weren't real live apes in the movie at all?'"
Such verisimilitude is not cheap, with each five seconds of film costing between £30,000-£35,000. Cost is something to bear in mind, says Reeves, but it should not dominate.
"At a certain point you are caught up in the realities of the filmmaking and you are telling the story. I was aware of it but at a certain point when you are shooting you kind of have to put it aside. If I had closed my mind to what was going to happen on the set because I thought well, it is just going to be these three shots then I wouldn't have discovered most of the movie. That isn't how you make a movie."
New York-born Reeves, now 48, has been making films since he was a boy. When as a teenager he submitted his Super 8 efforts to a film festival in Los Angeles, two remarkable events followed. First, he met the young JJ Abrams, then just another Super 8 aficionado, now director of the new Star Wars. Second, Reeves and Abrams' efforts impressed Steven Spielberg so much he asked them to restore his own old Super 8 films.
"We couldn't believe it. He was one of our heroes." One of the films turned out to be an early teenaged version of Close Encounters.
Abrams and Reeves have been friends, and sounding boards for each other, since. Abrams produced Reeves's breakthrough hit, the science fiction thriller Cloverfield (2008). "We're always talking," says Reeves, who says his pal will do "amazingly well" with the new Star Wars now shooting.
"It's funny, as kids Planet of the Apes and Star Wars were two of the movie franchises that we were obsessed with. I know talking to him now how excited but nervous he is. Each time I see him nervous like that I know it's because he's going to do something really great."
After Cloverfield, Reeves took on the job of turning the beloved Swedish vampire movie, Let the Right One In (2008), into Let Me In (2010), starring Kodi Smit-McPhee, who also appears in Dawn. By showing he was equally at home with visual effects and drama, Reeves was deemed the perfect fit to take on Apes. After a short break, he will start writing the next instalment.
It had been thought the new series would be a trilogy, but Reeves sees the tale as a generational, Godfather-type family story, with Caesar as a kind of "Don Corleone of the apes". So there might be another film after the third instalment? "At least."
Serkis's performance will reignite the debate as to whether there should be a new category at the Oscars to recognise the march of motion capture technology. Reeves disagrees.
"They've got two categories that already apply - best performance by an actor, which Andy certainly qualifies for in my mind, and best visual effects in a motion picture, and there's no question Weta deserves credit for that."
There is still confusion around performance capture, says Reeves. At its heart it is an actor's performance.
"I had worried that motion capture involved some technical process that was going to create an obstacle between me and the actors. The big revelation for me was actually that the technical part is about recording the performance, that's all. We had almost no technical conversations whatsoever. It was all about character."
Dawn of the Planet of the Apes opens in cinemas tomorrow.