To play Spider-Man, Tom Holland drew on his background as a dancer to athletically throw himself into spectacular stunts. But "Spider-Man: Homecoming" also features an entirely computer-generated digital double -- and not always for the reasons you might think.

"Homecoming" is available for home viewing in the US now, and swings onto Blu-ray and DVD in the UK on 20 Nov. To find out more about creating computer-generated (CG) characters for effects-driven blockbusters like "Homecoming", I caught up with Theo Bialek, visual effects supervisor at Sony Pictures Imageworks, which was responsible for the film's climactic fight scenes.

To create each action-packed sequence, "Homecoming" director Jon Watts filmed Holland and Michael Keaton, who plays the villainous Vulture, on sets and against green screen. These "plates" -- shots of actual actors and stunt performers on real sets -- were composited to create a finished shot, or used as reference by Bialek and his visual effects team to build their CG characters. Of the 300 or so shots Bialek's team created for the film's finale, more than a hundred involved an entirely CG Spider-Man.

CNET: How do you decide when to use plates featuring real footage of the actors and when to use a digital double?

Bialek: There's some shots that were plates, and then they switched them to all-CG because they wanted to do an action that was different to what they shot. But generally we push the client to use a plate as much as we can, just because when you have a lot of all-CG shots next to each other it's harder to pull that off and still keep the audience grounded and not pulled out from it. It's always nice to have shots in there that are real to bookend the CG, or interspersed, to keep us anchored.

Sometimes there'd be a plate shot and they didn't like the way the ears came out of the mask so we would replace the head, or just do the shot all-CG.

Wait, you'd literally completely replace Spider-Man's head because they didn't like how the ears looked?

Bialek: Yeah, the ears stick out from the mask. Or if the suit just didn't look quite right in that particular shot because of a stain on it or something like that. That was rare, though.

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How did you draw on the real footage of the actors, and why did you look at footage where they messed up a stunt as well as when they got it right?

Bialek: A lot of times we'd look at something we animated and it just didn't feel right, and it was hard to put your finger on exactly why because he was doing all the things he should be doing. Then you go and you look at the reference and you realize his knee knocks over and hits this other knee and it kind of buckles in on itself when he's taking his weight or when he steps back. You wouldn't always think to put that in.

It would be the way that he would kind of relax his shoulders, or when he was talking how he might try to use his hands a little bit -- just keying in on those mannerisms.

What kind of individual mannerisms did you identify in Tom Holland and Michael Keaton?

Bialek: For Tom, the way he runs, his gait. When you look at the mo-cap you can tell it's Tom. It's the way his shoulders drop and the way he holds himself, his posture.

Keaton, not so much. He was a little more difficult because when we're doing him all-CG, he would be in his exoskeleton wingsuit. His mannerisms wouldn't be translated to his suit because you wouldn't move naturally in a suit that's confined to an armature. We would key into how he paced himself because the character he's playing doesn't move super-fast. His suit allows him to fly quickly, but he wouldn't react faster than a normal person.

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Spider-man wears a mask, so you can't see his face in action scenes. From an effects point of view is that a blessing, or is it difficult to make a blank face relatable?

Bialek: That's a huge blessing. It's so much easier to pull off. For almost all of the shots in the beach battle Spider-Man has his mask off and he's got a CG face, but it's at a distance and there's a lot of smoke and firelight flickering so the facial animation and the hair simulation didn't have to hold up to being a close-up shot.

When he's wearing the mask and there's dialog, we still animate it underneath the mask. When he's breathing hard, we have the mask kind of suck in and suck out. Those are things you want to keep it relatable, so it doesn't feel like a puppet or something.

For more on "Spider-Man: Homecoming", find out why the toughest effect was Tony Stark's hair and why director Jon Watts knows how Peter Parker feels.

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