If you mimic your opponent (in a subtle way), they are more likely to believe you.
For example, if they are sitting cross-legged, wait a few seconds and cross your legs too. And make sure that what you are doing is not too obvious.
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To gain trust and build rapport, you need to hear out what the other person thinks without interrupting or disagreeing.
Try asking open-ended questions, like: "Why do you think that?"
...while you listen. This makes the speaker's arguments less persuasive, which makes your opinion look strong.
Fix the speaker in your sight as soon as they start speaking.
Repeating an argument back to the speaker can develop trust by proving that you're listening.
Try paraphrasing what you understand, using: " so you're suggesting ... because... ?"