I think the question to some extent is about the stage you sign an NDA. While it's perfectly normal to sign an NDA when you start a contract, I wouldn't sign one before initial conversations unless I strongly trusted the client, because without knowing the scope of the project you have _no idea_ what's going to be covered by the NDA and it may include things that cause you significant problems down the line.
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I think the question to some extent is about the stage you sign an NDA. While it's perfectly normal to sign an NDA when you start a contract, I wouldn't sign one before initial conversations unless I strongly trusted the client, because without knowing the scope of the project you have _no idea_ what's going to be covered by the NDA and it may include things that cause you significant problems down the line.