Michael Cohen, President Donald Trump’s former lawyer, has withdrawn his offer to testify before Congress, citing what he believes to be safety concerns. Cohen may or may not ever testify — he is to report to federal prison in March. Either way, the dustup over Cohen may presage a season of congressional investigations of the executive branch (and of industry) such as we have not seen in recent years. Congressional investigations are peculiar creatures, both substantively and procedurally — part law, part political theater, part constitutional struggle. For persons and businesses who are the subject of a congressional investigation, and for the lawyers who advise them, a seemingly anodyne tool — staff depositions — has received new life under the new majority and could make congressional investigations faster, more penetrating and more dangerous.