This is set before the show actually starts
Yes, I have done this before and once they realised I was fine everyone found it very funny
When Spencer first joined the BAU he was nervous. Every time in his past that he has joined a group he has been relentlessly tormented and ridiculed. Now, he is joining a team where a requirement in that you pass a physical and agility exam. An exam Gideon got waved for him so he had a fighting chance of getting in. Which means that he will almost certainly be the weakest on the team.
Despite his concerns, Spencer was pleasantly surprised. Gideon introduces him to the group as a whole, then takes him to Hotches office to formally introduce him. Turns out Hotch isn't as scary as he looks, in fact, he's quite nice. He shows him to his desk in the bullpen, before him and Gideon leave him to settle while the rest of the team are on their break. JJ is the first to return, she smiles and says hi to him, offering one of the cookies she'd bought and Reid manages to mask his mistrust, after all, in his experience pretty girls are sweet and kind only until you trust them enough for them to stab you in the back. Or tie you naked to a goalpost.
The next person he meets wasn't there when Gideon introduced him to the team. She wears brightly coloured clothes and doesn't really look like an FBI agent.
"Hi sweetie, are you the newbie?" She asks brightly. Spencer is slightly taken aback by the bubbly and upbeat persona, having expected to only come across serious, suit wearing men who look down on the fact that he has no hope of lifting a barbell over his head.
"Uh, yeah" he replies, trying to figure out why she's now just staring at him.
"Am I allowed to know your name?" She smiles and Spencer feels like a moron for not realising he should introduce himself.
"I'm Spencer Reid" he sputters, the strange woman smiles kindly at him "nice to meet you, I'm Penelope Garcia, but you can call me goddess if you like" and once again Spencer doesn't know how to react, but he smiles, because he has a feeling that this woman is probably the person actually in charge of the BAU, even if Hotch thinks he is. Garcia helps him set up his computer, and ends up giving him a crash course in its basic functions. Hotch brings him out a duplicate file to attempt to check he knows how to fill them in properly. Finally, he meets the person he was most afraid of from the beginning.
"Hey man" the darker man calls, raising his hand to shoulder height. Spencer doesn't mean to flinch, but when he does the man changes his approach. "Hi, I'm Derek Morgan" he says with his voice even "nice to meet you man"
"Nice to meet you" Reid replies meekly before the two turn back to their work.
It took about 2 weeks for Spencer to let his guard down around the team, other than Gideon. But as he did, and he got to know them, he realised quite how wrong his expectations are. Hotch, although always serious, also has a sense of humour. Even when he accidentally manages to hit him with one of his 'rockets'. JJ somehow is a pretty girl on the outside, but on the inside is one of the girls who liked to mother him when he first started highschool at age 9. Even Derek, who he assumed would take every opportunity to bully him about his physique, his age, his personal life, anything, turns out to be a good person. It takes him a while to get Dereks sense of humour, but as it turns out he only teases people he likes, and avoids jokes that he can see upset anyone. He doesn't even mind when Spencer makes a joke in return, he damn near encourages it in fact. For the first time, well, ever, he has a whole group of friends.
When Derek goes to Chicago for his mother's birthday (a yearly tradition so he's told) he hadn't expected to miss him. The office is boring without him. He misses his jokes, the way he messes his hair up, and although without Derek sneaking him files he doesn't have as much work to do, there's no point when there's no one to mess around with he can't really enjoy it. When the day finally comes that Derek is coming back, he's pretty excited.
Derek returns to work after 3 days of visiting his mom expecting to come in, sit down, have a leisurely conversation with his coworkers and then be given a stack of files. He doesn't expect to walk in and see everyone crowded around Reid's desk, with him sat in his chair, ice pack being held to his head by JJ. Hotch is stood off to one side, arms folded, Gideon is sat on Reid's desk, smiling fondly at a slightly pouting Reid.
"Hey guys, what's goin' on?" He asks, causing the others to turn to him.
"Hi morgan!" Spencer chirps, trying to stand, only for Hotch to glare at him until he sits back down. Everyone says their hellos and exchanges pleasantries before Derek can get an answer to his question.
"Spence got a little over excited, started jumping around, and managed to slip on a marble" JJ explains, Gideon surpresses a smile
"He hit his head on his way down" Hotch finishes "we're waiting for first aid to come check him over" Derek can't quite wrap his head around it
"What got you so excited you were jumping?" He laughs
"I jump a lot when I'm excited" he defends
"Ok, so what got you excited" and Spencer just ducks his head
"Because he gets to see his friend again" JJ smiles, as does Hotch and Gideon.
"I knew you'd make friends" Gideon laughs, and he's right. In a few months he's gone from being afraid of Derek, to being so excited to see him that he stims himself into a head injury.