I've never tried that sort of faking. I suspect that if I wanted to do it, I might be good at it. It's more like the creative writing of fiction than it is like lying to someone to their face, and trying to play a part in the flesh, with limited time between lines of dialogue.
I was involved in a really nasty one, though. I was (still am) part of a close-knit Christian e-mail Listserv, most of us of the Anglican Communion (that is the "topic" of the List), mostly from the US, Canada, England, Australia, and New Zealand, very occasionally from somewhere else. We observe each other's birthdays when possible, have a Christmas gift exchange with random Listlings selected to send by snail a gift to whose-ever name they've "drawn", praying for each other, baring our hearts and souls to the group. We trusted each other, often getting together in the flesh in small groups, especially on the East Coast of the US, and when someone had some necessity for traveling outside their own area, they'd check on List to see who else might be at their destination or along the route with whom they could get together. It's not a large group (especially not these days: I'm talking about the late nineties, now) compared to this board: in those days, there were maybe fifty to a hundred or so posting, some more frequently than others, and the posts could get quite long and detailed, and some unknown number, quite variable, lurking or temporarily or permanently absent according to life circumstances.
One of our valuable, loved and respected members was a relatively young woman, divorced after an abusive marriage, with a restraining order for her ex to stay away from her in the Real World. She wrote beautiful prayers. I'll refer to her as Jane. There entered a newbie to the group, a relatively young woman, recovering from an abusive marriage (she went into detail) and afraid of her ex. I think she used the name Sharon, but I'm not sure, it's been a while. Naturally, with so much in common, she and Jane entered into private, offList correspondence. After some weeks, it turned out that "Sharon" was a "sock-puppet" for Jane's ex, who was stalking her. I'm not even sure that the stalking laws apply to e-mail. He was banned from the List, after the revelation, but kept coming back with fake addresses. One of our members was an major expert in internet, and he took the part of detective, running down each address, as they tried to join the List. Frequently, he did get back, and was seriously disruptive and hateful to every one of Jane's friends. He'd known when they were married how important the group was to her. Eventually, Jane left the List, seemed to her that it was the only thing that she could do. That man was a superlative creative writer-- he got back with several more widely variant identities before he gave up, but he'd already taken Jane's community and comfort away from her. Some of his posts were grossly horrible, and one got into reading them before realizing that that was another mask. I think he had to have been psychotic. I don't know what ever happened to either of them.
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Asperges me, Domine