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It is common for players to show each other their hands, either intentionally or accidentally. Most games are dealt face up anyway. I answer questions accurately, if asked and if I think it offers no exposure of my skill set. Dealers always give players advice on plays and are blatantly wrong a high percent of the time. I will never tell a player to do something that is a departure from basic strategy even if I know from counting that the condition of the shoe warrants it.
I do not volunteer advice.
Correcting a dealer who tells a player how to make a play merely points out my skill level. If the player ask me then I always say "well it is your money but this is what I would do".
If I'm playing, I'm going to focus on what's best for my hand, and not really disturb someone else. If they ask, I might say what I would play, but I wouldn't want them to hold me responsible for them losing. I know it's the run of the cards, but some players like to take their loss out on someone, and I'm not interested in getting into that. It ruins a good evening.
Stealth, I agree with not volunteering advice. I mean we're all here to win right, so why help other players who don't ask for help? Like you stated, it is different if the plays just comes out and asks for your advice.
No, not usually. If I'm there with friends, or someone I know as part of a group trip, then I might offer tips about basic play to keep them in the game longer. If its a stranger then no, because I don't know who they are and whether they actually need the help or just want to gauge my playing style.
I do have other players offering me advice on certain plays that they think "changes the flow of the cards" or "you took the dealers bust card". This sort of nonsense comes with various plays that are a departure from basic strategy (the norm) due to having card counting information that dictates the change.
As an example, hitting 12 versus a dealer up card of 5 will bring loud complaints. There are several more, A7 versus a dealer 10, 97 versus a dealer 10, etc. All are advantage plays based on perfect basic strategy and/or card counting.
Be aware that what you think is a bad playing decision may be an advantage play!
It really annoys me if someone at the table sees me make an error and corrects me. I'm not there to get schooled. I'm there to have fun and if I make errors occasionally, it is on me. I don't want help on the odds or anything else. I do what I do just for fun!