Collins obeyed.

Not voluntarily, but because he was unable to resist the domination of the detective's will.

Also, a terrible fear had gripped his heart, producing a terror that sobered him and gave him command of all his faculties.
"Who are these men ?" inquired Britz, nodding toward Cooper and Fanwell.
"Friends of mine," growled Collins.
"I wish to speak with you, Collins," said the detective.

"Do you want them to remain ?" "I do." "You prefer to have witnesses present ?" "I wouldn't talk to you without them," said Collins.
"But I want to give you an opportunity to explain certain things in connection with Mr.Whitmore's death." A crafty expression overspread Collins's face.
"Look here, officer!" he exclaimed, a weak smile on his lips.

"I'm no boob!" Obviously, he meant this lapse into the slang of the Tenderloin to convey his intimate knowledge of police methods.