In The Lord of the Rings Andy Serkis plays Gollum and wears a big blue suit which with computer imaging he becomes Gollum and i could not write it better than this quote explains how he makes gollum come to life

Few of the hundreds of characters within The
Lord of the Rings are as fascinating as the tormented Gollum. Though he is
entirely computer-generated onscreen, his haunted personality is brought vividly
to life through the extraordinary talent and performance of Andy Serkis.
Serkis's voice, body, and facial movements served as the electronic blueprint
for the technical wizards who conjured the character seen in the film, and it
was his physical presence — later removed digitally — that served as the foil
for the actors playing Frodo and Sam. The behind-the-scenes story of a
breakthrough cinematic achievement is told in this book.

as the quote states this man really is the character gollum, he makes gollum so scary and makes him really come to life, and makes the movie and character what it is.

the blue suit makes it so that they can put on the gollum costume with all computer generation.