She has grown/ been trained by my sister into a pretty placid adult dog, but when young she had some weird quirks and phobias.
And still does do some strange stuff: when she has finished eating she picks up her dinner bowl, takes it out of the kitchen, plonks it in the middle of the living room floor, then returns to the kitchen. Why? Who knows.
Border terriers are generally nuts as a puppy and into adolescence. From about 3 years they tend to flip a switch and become very mild mannered and quiet. By about that age they often ignore other dogs and possibly humans too. Most BTs I've met have been indifferent to dogs but 50:50 indifferent to humans too. They're bred to run with much larger hounds so can't be reactive or the pack would tear it apart. That's why they behave like that mostly as adults. Hard wired!
They're also possibly the most intelligent terrier breed. That's according to a couple of behavioural studies on dog breed intelligence. That's why they do actually train up well. They do occasionally have the terrier deaf ear but mostly pretty good.
Mine walked off the lead in countryside from a puppy about 10 weeks. At our puppy training class on her first session she learnt paw both left and right paw on command, sit, lie down and stay. In fact the stay involved setting her down one end of the hall, walking back to the other telling her to stay. Then after 10 seconds of her being settled I called her to me. She did it straight away. All the other dogs either ignored the stay, had to be held until the recall or got distracted by the hall of dogs on either side. The trainer was amazed and the other owners too, if a little annoyed. We stopped going after 2 or 3 sessions, there was no point she could do it!
There was this village fete once that had a sheepdog and ABC agility display. Afterwards they invited dog owners to give it a try. She was the only non collie that got close to completing it. I guess dog owners see their dog as perfect. Mine isn't just it trained itself very well, not has training it. Also, it trained us well too!