Learn Portuguese (Portugal): Ele está a agir como se fosse um espião.

The phrase como se (as if) always triggers the Imperfect Subjunctive tense because it sets up a hypothetical or imaginary situation. Here, he isn't actually a spy, so we use fosse (the imperfect subjunctive of ser) instead of a standard indicative past tense.

Why use the verb ser (fosse) here instead of estar?

We use ser because it links to a noun defining an identity or profession (um espião). Even though he is only pretending in this scenario, the hypothetical role itself requires ser. You would use the subjunctive of estar for a temporary condition, like como se estivesse doente (as if he were sick).

Why is it está a agir instead of está agindo?

In European Portuguese, ongoing actions (like "is acting") are formed with estar a followed by the infinitive of the verb (agir). The form está agindo uses the gerund, which is standard in Brazilian Portuguese but rarely used in Portugal.

Do I need to repeat the pronoun and say como se ele fosse?

You can, but it is not required. Portuguese frequently drops subject pronouns when the context is clear. Since we already established Ele at the beginning of the sentence, the verb form fosse is enough to show we are still talking about him.