RBS's outgoing finance director told the Bank of America Merrill Lynch Banking & Insurance CEO (chief executive officer) Conference that there is "more work to be done" on overhauling its North American bank, which has a presence in 12 states.
According to slides published by the bank, Mr Van Saun said there is an "intense focus on improving returns" at Citizens as RBS prepares for an initial public offering in late 2014 or 2015.
It is focusing on costs, concerned that there is a "sizeable gap" between the return on equity generated by Citizens and its peers.
RBS revealed in February that it plans to sell a 25% holding in Citizens, as it came under pressure from regulators to improve the group's capital buffers.
Mr Van Saun told delegates at the conference the flotation of Citizens will boost awareness of the brand in its home market while giving RBS flexibility.
There are "clear lessons to be applied to Citizens IPO" from the flotation of Direct Line Mr van Saun said, speaking just days after another share sale cut RBS's stake in the insurer to just 28.5%.
These include the efforts of Direct Line's management in restructuring the business and a "relentless focus on the customer and financial performance".
Mr Van Saun will become Citizens' chief executive next month. He will be replaced by Nathan Bostock.