If you’re a U.S. citizen, you may need to apply for a work permit to work in Canada. If you’re in certain professions, such as in a film crew or a convention organizer, you don’t need a work permit.
If you reside in Canada for more than 183 days in a calendar year, you are required to file an income tax return in Canada and declare all your income regardless of the sources. If you file a 1040 federal tax return in the United States, you are exempted from paying tax on the income you earned and paid taxes on in Canada.
If you work in Canada but don’t live there, unless you applied for an exemption of tax withholding or your employer is a U.S. company, you are required to file a Canadian tax return and pay tax on the income you earned in Canada.