Question about Google Spreadsheets And Google Apps Script

[XcelMan]

So I decided to teach myself how to write Google Apps Script codes in the months ahead. Here's my questions. If I have a Google spreadsheet in Google Drive's "My Drive" folder, is it possible to use it with Google Apps Script to make changes to another Google spreadsheet in Google Drive's "Shared with Me" folder without any authentication issues? And will the script interfere with anyone who's updating the Google spreadsheet in real-time from "Shared with Me" folder while the script is running?

Thank you for your responses.