The subject may have the affected semantic role that is typical of the direct object when the verb is intransitive:
1a. 老奶奶摔倒了。
Lǎonǎinai shuāi dǎo le .
(The old grandmother fell down.) [S as affected]
1b. 书掉到地板上。
Shū diào dào dìbǎn shàng
(The book fell onto the floor.) [S as affected]
1c. 牛肉在炖着。
Niúròu zài dùn zhe
(The beef is stewing.) (S as affected)
1d. 妈妈在炖牛肉。
Māma zài dùn niúròu .
(Mother is stewing beef.) [S as agentive, Od as affected]
Both the subjects in (1a, b) have the semantic role of the affected. Please compare (1c) and (1d). The verb 炖‘dùn’ (stew) is transitive in (1d) and the direct object has the semantic role of the affected. When the verb is used as an intransitive verb in (1c), the subject (the object of (1d)) still has the affected semantic role. The difference between agentive and affected subjects lies in the fact whether an element of causation or volition is present. Some verbs allow both interpretations: 他突然笑了起来。‘Tā tūrán xiào le qǐlai’, (Suddenly he started laughing). In the case of 笑了起来‘xiào le qǐlai’ (start laughing) as an involuntary action (e.g., after being amused by a funny thing), the subject referent has the assignment of the affected role, but if the subject deliberately executes the action of laughing to cause attention, then it is agentive. A volitional adverbial (e.g., 有意的‘yǒuyì de’ (intentionally), 故意的‘gùyì de’ (deliberately)) can only occur with an agentive subject, as in 他突然故意的笑了起来。‘Tā tūrán gùyì de xiào le qǐlai’ (Suddenly he deliberately started laughing).
[1] Loar, J. K. (2011). Chinese syntactic grammar: functional and conceptual principles. New York: Peter Lang.
