An employer may require an employee to work overtime under the following circumstances as mentioned in Section 60A(2) of the Employment Act:
"(2) An employee may be required by his employer to exceed the limit of hours prescribed in subsection (1) and to work on a rest day, in the case of:
(a) accident, actual or threatened, in or with respect to his place of work;
(b) work, the performance of which is essential to the life of the community;
(c ) work essential for the defence or security of Malaysia;
(d) urgent work to be done to machinery or plant;
(e) an interruption of work which it was impossible to foresee; or
(f) work to be performed by employees in any industrial undertaking essential to the economy of Malaysia or any essential service as defined in the Industrial Relations Act 1967:"
That's what the law says and the Labour Department has the right to decide should there be dispute over those above mentioned circumstances. Normally, it would be better during the interview to ask the candidate whether the person is willing to work overtime and put in a clause in the appointment letter that "..the employee may be required to work overtime and the employee shall not unreasonably refuse to do overtime".
So, in your case, you can be guided by subsection (2) above.