Translation:I thought you had talked to him.
The verb in the principal clause determines the use of the past subjunctive in the subordinate clause. This is formal Portuguese, so there are always exceptions to the rule.
Using the past subjunctive probably indicates that what you expected to happen (talking to him) didn't occur.
Yes. Whitlam's Modern Brazilian Portuguese:
"Eu pensei que" can be followed by the indicative tense when there is a degree of certainty in what you are thinking.
• Quando ele disse isso, eu pensei que ele era um cara sério.
"Eu pensei que" is followed by the imperfect subjunctive when what you were thinking turns out to be mistaken.
• Eu pensei que ele fosse um cara sério, mas acabei me decepcionando."