You have no extra time and no extra resources but is asked by the CEO to add 20 extra features, what do you do?

First, I will try to figure out how important these 20 extra features, whether some or all of them are 'must have', 'should have','could have' or 'nice to have'. I would totally rule out any 'could have' or 'nice to have'. A project/program manager must have courage and conviction to say 'no'. If some or all of those features are 'must have' or 'should have', then I will see what features in current project/program can be dropped in favor of the new features. use creative project/program planning or use overtime as much as possible without adding extra risk. If none of the above is true, then extra resources or time must be negotiated. Through analysis of the situation and clear communication is key to achieve this objective.

It's always about trade offs. Add the 20 requested features to the list - then sit with stakeholders (marketing, engineering, etc) and sort the list by priority and by ease of implementation. Customer must haves should come first and these hopefully were articulated when the core product was conceived and started. Include in the list the level of completion of the features. Completed ones are done, move them below the line. Review the remaining - which can be completed in remaining time with available resources and which have to be dropped. Be the strong project manager and present to management. If the company only wants yes persons with no critical thinking skills, you probably don't want the job anyway.

For SW projects, we have to freeze requirements, scope, they can not change after a certain phase. Else developers has to start the design phase. I will take the 20 items to my dev team and ask what can be accommodated with minimal changes to existing plan. Ultimately business needs over-write all others.