Can I call methods on an object represented by a string variable?
Twig's attribute function allows you to set the method as a string and not the object. The object has to be an actual object, even with that function.

{% set object = craft.entries.order('postdate').last() %}
{% set method = 'title' %}

{{ attribute(object, method) }}


The only workaround I can think of is to link your object-representing string to an actual object in your template. But this makes little sense as you probably wouldn't have set it as a string to begin with, hehe. Anyways:

{% if string == "craft.entries.order('postdate').last()" %}
    {% set object = craft.entries.order('postdate').last() %}
{% endif %}

{{ attribute(object, method) }}