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In my humble opinion, a young rjl's favorite 3 flirtation tactics are as follows:
Wincing and stretching out his shoulders until Sirius offers to give him a shoulder massage
Writing silently about Sirius in his journal (it's a diary) alone in the corner of the common room, stealing looks at Sirius until he looks back, and then looking away so quickly he gets whiplash
And finally—his boldest, and most effective tactic—looking up at Sirius and batting his big brown eyes at him
Please share other Remus Lupin flirtation tactics in the tags !!