Comme si (as if) follows the same rule as si: never the conditional after it. Use the imparfait for something at the same time, and the plus-que-parfait for something earlier:

• Il parle comme s'il savait tout. — He talks as if he knew everything.
• Elle me regarde comme si elle ne me connaissait pas. — She looks at me as if she didn't know me.
• Il sourit comme si rien ne s'était passé. — He's smiling as if nothing had happened.

Two set phrases: faire comme si de rien n'était — to act as if nothing had happened, and c'est comme si c'était hier — it feels like yesterday. Before il, it becomes comme s'il.